Energy distributions from decomposition of a complex H 2O→OH+H on a simplified potential energy surface, as a function of total angular momentum: Comparison between classical trajectories and an RRKM-type statistical simulation

Abstract

The unimolecular decomposition of a model triatomic system similar to an excited water molecule was studied with the classical trajectory method. The decomposition channel was H 2O*→OH ( 2Π) + H. Energy term values for the different degrees of freedom were recorded for complexes at the centrifugal barrier and in the product state, using the total angular momentum as a parameter in the range 10–80 ħ. Results for the same type of system were also found from our RRKM-type statistical algorithm, which has previously been successfully applied to the unimolecular decomposition of excited H 2O formed in crossed beams (experiments by Buss et al.) and in LIF studies (experiments by Luntz et al.). The available energy is distributed between the degrees of freedom in an RRKM fashion, observing strict conservation of total angular momentum. By comparing the two sets of results a test of the fundamental assumptions in RRKM-type statistical theories, as well as the statistical behaviour of collision complexes is found. Satisfactory agreement between the dynamical and statistical results is found for all total angular momenta. At small total angular momenta the degeneration of orbital motion and molecular rotation into one-dimensional rotations, due to the alignment of the two angular momentum vectors, gives a statistical model with three “boxes”. The trajectory calculations show enhanced vibrational excitation compared to translational excitation for small total angular momenta. This effect increases with the total energy, and is proposed to be due to the anharmonicity of the potential energy in the excited diatom.